First Four Weeks of Development Flashcards
Embryonic Stage (timeline) and characteristics
Fertilization to day 56.
Zygote formation, cell division, implantation, organ formation, teratogen sensitivity (environmental agents that could cause congenital abnormalities)
first two weeks
TERATOGEN EXPOSURE= minor enough to be okay or spontaneous abortion
no congenital anomalies associated with the first two weeks
significance of weeks 3-8
PERIOD OF MAXIMAL SENSITIVITY TO ABNORMAL DEVELOPMENT - malformation of embryo may occur
(something can go wrong post 8 weeks but it would not be considered a malformation)
how long until the female oocyte degenerates? (time for sperm to fertilize)
degeneration may occur within 24 hours, however, sperm can last longer within the female
OVULATION OCCURS AT DAY 14-15
zygote
the diploid cell formed by the union of the haploid spermatozoa and the haploid ovum - genetically unique cell since half chromosomes from the mother, and half from the father
what is occuring once zygote has formed from fertlization
MITOTIC DIVISIONS- called cleavage divisions occuring.
NO INCREASE IN TOTAL CYTOPLASMIC MASS
DAUGHTER CELLS OF THESE DIVISION ARE CALLED BLASTOMERES ciliated cells and peg cells working -
BLASTOMERES
daughter cells of the cleavage divisions- mitotic divisions after fertilization - becoming smaller and smaller with each division
morula
product of cleavage divisons produced by the blastomers - around 12-24 sources
how long does the zona pellicuda stay in tact?
stays intact until about day 19, so about 3 days post-fertilization
when does the first cavity begin to develop in the in the embryo? what is it called?
about 4 1/2 to 5 days post fertilization
referred to as the blasoCYST- formed by the blastomere and we have an inner and outer cell mass
what has to happen for implantation to occur?
the zona pellucida must be broken down for implantation to occur
window for implantation to occur
6-7 days post fertlilization
The two cell layer in the blastocyst
- Trophoblast
2. Embryoblast
Trophoblast
Thin layer around the blastocyst which penetrates the endometrium over the embryonic pole
GIVES RISE TO PLACENTA
Embryoblast
INNER CELL MASS OF BLASTOCYST - will give rise to all tissues of the embryo
name of endometrium once implantation has occured
decidua
what does the trophoblast proliferate into and describe them
two layers
- cyrotrophoblast - mitotically active, inner layer of cells
- Syncytiotrophoblast- PRODUCES hCG- TO MAINTAIN CORPUS LUTEUM - TO OVARY - MAINTAIN HORMONES UNTIL PLACENTA IS FULLY FUNCTIONING - this is always positioned closest to the maternal blood supply
hCG
human chorionic gonadotropin hormone - feedback to the ovary of corpus luteum (hormones maintain the pregnancy until the placenta has fully formed)
Acts on the ovary
day 7-9 post fertilization
trophoblastic plate formation and invasion of uterine stroma by blastocyst
two most likely places of an ectopic pregnancy
ampulla of the uterine tube and the rectouterine pouch - abdominal cavity
after week one?
fertilization and implantation have occured
major events in second week of development?
- Formation of the bilaminar disc - two layers that arise from the embryoblast or inner cell mass
- completion of implantation
embryoblast divides into what two layers
describe them
- Epiblast which is the dorsal side of the embryo - related to the AMNIOTIC CAVITY
- Hypoblast which is the ventral side of the embryo - adjacent to YOLK SAC and called the primitive endoderm and does not contribute to the developing embryo
bilaminar disk
- epiblast
2. hypoblast
what type of tissue lines all cavities?
epithelium
sources of fluid in the amniotic cavity
- fetal urine
- maternal sources - early in development
- amniotic cells forming the membrane - small amount
yolk sac
cells from hypoblast form a membrane that lines yolk sac
small in humans and contains no yolk
NUTRIENTS LIKE FOLIC ACID VITAMINS A B12 AND E PRIOR TO PLACENTA
aids in primordial germ cells and hematopoiesis
nourished through LACUNAR NETWORKS early in development
primordial uteroplacental circulation
provides nutritional needs early in development and is achieved by the trophoblasic lacunae made by the syncytiotrophoblast eroding away to allow maternal blood cells
extra embryonic mesoderm
layer that forms betweent he yolk sac and the cytotrophoblast layer
will start to form small cavities and give rise to one larger cavity
chorionic cavity - how is it made and what are its components?
result of the extra embryonic mesoderm and divides it into two layers
- extraembryonic somatic mesoderm - lining trophoblast and amnion
- extraembryonic splanchnic mesoderm- lines yolk sac
components of the placenta
Fetal components = what is derived from the chorion (3 components)
Maternal = the decidua - endometrium at sight of implantation
chorionic villi
extend outward from the chorion and contact maternal blood through the syncytiotrophoblast
Day 14 in development
the embryo has the form of a flat, bilaminar disc, ovoid in shape and in a localized are of the hypoblast, cells thicken and become more columnar and form the PRECHORDAL PLATE
prechordal plate
at the rostral end of the embryo and indicates the future site of the mouth - close to the oropharyngeal membrane
has a role in the induction of the forebrain and contributes to the mesoderm of the head region
end of week two?
‘Rule of Two’s’
Embryo is fully implanted
Trophoblast has had a period of growth greater than the embryoblast
More detailed description of what has occurred after week two
rule of two’s
- embryoblast = epiblast and hypoblast
- Trophoblast = cytotrophoblast and syncitiotrophoblast
- cavities = amnionic and chorionic (3 components)
- two yolk sacs - primary and secondary
- extraembryonic mesoderm splits into two layers = somatic and splanchnic
what do we have at the VERY BEGINNING OF beginning week three of development
Bilaminar disc with primitive streak resulting in bilateral symmetry
Significant event in the third week of development
cell proliferation and migration converts the bilaminar disc to the TRILAMINAR DISC AND GASTRULATION PRODUCES THE THREE GERM LAYERS
structures and terms associated with the third week
Trilaminar Disc
Gastrulation to produce three germ layers
primitive streak - site of cell migration
primitive node- elevated area at the cephalic end of the primitive streak
T/F epiblasts are responsible for making all three germ layers
TRUE
Epiblasts migrate through primitive streak and most ventral now is endoderm
mesoderm in middle
then original outer of epi becomes ectoderm
cardiogenic mesoderm
most cranially migrating cells form the cardiogenic mesoderm and goes ABOVE (cranial) to future position of the oropharyngeal membrane
lateral migrating mesoderm during gastrulation
paraxial, intermediate, and lateral plate mesoderm
day 16
will have a difinitive endoderm formed from epiblast cells that replace the entire hypoblast layer
what does the endoderm take place of
hypoblast layer which has degenerated and now is endoderm formed from migration of epiblast through primitive streak
notochord development
the ectoderm and endoderm are adhered together at the oropharyngeal and cloacal membranes so the AXIAL mesoderm forms a long cord along the cranial-caudal axis and develops into the NOTOCHORD
importance of the notochord
SIGNALING CENTER AND INDUCTIVE SIGNALS
- vertebral column and base of skull develop AROUND IT
- stimulates conversion of overlying surface ectoderm into neural tissue -CNS
- transformation of mesodermal cells of the somites into vertebral bodies
Adult remnant of notochord
nucleus pulposus of the inter-vertebral disc
term for zygote as it is migrating through the oviduct
morula (during week one around day 3-4)
hatching
refers to the degeneration of the zona pellucida during week one of the free blastocyst - right before implantation
what does the neural tube give rise to?
CNS
brain and spinal cord
ectoderm derivatives
surface ectoderm (many derivatives)
neural plate (neural ectoderm)
neural crest cells
ectodermal placodes
columnar epithelial cells of the neural plate
neuroectoderm
neuroepithelium
what day does the cranial neuropore close (approximately) in humans?
day 24
what day does the caudalneuropore close (approximately) in humans?
day 26 (might not be exact but the caudal pore ALWAYS CLOSES AFTER THE CRANIAL)
causation of ectopic spinal nerves?
dysfunction in the notochord as it is a signaling center and is crucial in the development and organization of the spinal cord
path of neural crest cells
as the neural tube is folding, neural crest cells are migrating into the mesenchyme on each side of the tube and go to various locations into neural and non-neural cells
give rise to the cells of the peripheral nervous system as well as other important structures
key points of neural crest cells
- derived from ectoderm related to the neural tube
- often called the fourth germ layer
- must migrate from their site of origin and undergo ectodermal mesenchymal differentiation
- very vulnerable cells
what makes neural crest cells so vulnerable
must migrate from their site of origin (from ectoderm related to neural tube) and undergo ectodermal to mesenchymal differentiation
two types of ectoderm
- surface ectoderm
2. neural ectoderm
Derivatives of surface ectoderm
ALOT
- Epidermis, Nails, Hair, Subcutaneous glands, mammary glands, anterior pituitary, enamel, lens of the eye
derivatives of neural ectoderm
CNS
Retina
Posterior Pituitary gland
Pineal Body
intraembryonic mesoderm
mesoderm and its derivateives the embryonic mesoderm on each side of the neural tube and notochord organizes into three distinct longitudinal columns of cells 1. Paraxial - adjacent to axial 2. Intermediate 3. Lateral plate mesenchyme
paraxial mesoderm
next to axial mesoderm
organizes into somitomeres in the head region
from occipital region caudally somitomeres organize into somites
AGE OF EMBRYO IS CORRELATED TO THE NUMBER OF SOMITES
What does each somite form?
scleretome
myotome
dermatome
paraxial mesoderm in the head region?
from occipital region and down?
somitomeres
somites (occipital and caudal)
scleretome
from paraxial somite and forms the axial skeleton or segmental bone
dermatome
from paraxial somite and forms the epidermis/ dermis/ connective tissue of the back
myotome
paraxial somite and forms the skeletal muscle of the back, limb, and body wall
intermediate mesoderm
gives rise to the urogenital system
functionally divided into urinary system and genital system
division of the lateral plate mesoderm
somatic (parietal) mesoderm
splanchnic (visceral) mesoderm layer
somatic/parietal mesoderm
this faces the ectoderm and in combination will form the body wall
*in relation to body wall
splanchnic (visceral) mesoderm
this faces the endoderm and in combination with it will form the gut walls
*in relation to an organ
what will the cells lining the parietal and visceral layers of the body cavity differentiate into?
specialized simple squamous epithelium called MESOTHELIUM
what will the single intraembryonic body cavity give rise to?
- pleural cavities
- pericardial cavities
- peritoneal cavity
what causes the longitudinal folding of embryo (where is it occuring)?
folding due to brain development and causes the head and tail to fold ventrally
on the longitudinal or median plane (cranial - caudal)
what causes the lateral fold?
due to the growth of the somites
occurs on the horizontal plane (lateral body fold)
What week of development is the endoderm (epithelium) folded into the body cavity?
fourth week of development
What does the endoderm give rise to?
- epithelium of the gut tube
- Liver, gall bladder, pancreas
- epithelium of respiratory system
- epithelium of SOME of the glands and structures of the head and neck
cranial head fold
endoderm is incorporated into the embryo and forms the FOREGUT
- septum transversum
- primordial heart
- pericardial cavity
- oropharyngeal membrane
tail folding
endoderm lining the yolk sac is incorporated as the hindgut
- primitive streak
- cloacal membrane
- connecting stalk
lateral folding
GROWTH OF SOMITES
-forms body wall
endoderm is incorporated as the midgut - in communication with the yolk sac
